112 



DISCOVERY 



of the geological periods fairly closely. Uraninite of 

 Eocene (Lower Tertiary-) age gives a ratio indicating 

 that 70 million years have clajised since it crystallised. 

 For the Permo-Carbonifcrous igneous rocks of three 

 different locaHties, ages of 300 to 340 million years are 

 obtained. For the Middle Pre-Cambrian, a group of 

 periods which includes at least two great epochs of 

 igneous activity, ages varying from 900 to 1,120 million 

 years have been deduced. The oldest mineral hitherto 

 analysed appears to be a zircon from the Lower Pre- 

 Cambrian of Canada. This gives an age of 1580 

 million years, but as it is based on a single analysis, it 

 can only be considered to give an approximate estimate 

 of the time that has elapsed: However, the figure is 

 probablj' substantially correct, as it is supported by 

 helium-ratios which, as usual, give less than half the 

 period represented by the corresponding lead -ratio. 



The data so far collected are summarised below in 

 tabular form, and it will be noticed that in some cases 

 the lead-ratio can be used, as in Mozambique, for deter- 

 mining the geological position of rocks which yield their 

 age to no other method of investigation. There is, for 

 example, no doubt that the zircons from Mozambique 

 point to Middle and Lower Pre-Cambrian ages for the 

 rocks from which they were respectively obtained. The 



correlation of the Pre-Cambrian formations in different 

 parts of the world has long been one of the most difficult 

 problems with which the geologist has been faced. 

 The method of lead-ratios, however, has already done 

 good service in leading the way towards a world-wide 

 solution. 



In conclusion it may be pointed out that the age of 

 the earth is likely to be greater than, say, 1,600 miUion 

 years, for the oldest known igneous rocks are them- 

 selves intrusive into sedimentar\- formations which in 

 turn must have been derived from still older igneous 

 rocks. The latter may possiblj- have been the original 

 crust of the earth, but of this no certain trace has ever 

 been detected. Now, as in Hutton's day, geologists 

 can still find " no vestige of a beginning." Astro- 

 nomical considerations, however, have shown in recent 

 years that the new demands on geological time are not 

 too high. The movement of the solar system across 

 the void of space from its supposed birthplace in the 

 Milky Way is a journey for which something approach- 

 ing 3.000 million years is a dynamical necessity. 

 Supporting this figure is an estimate by Dr. Harold 

 Jeffreys of the age of the solar system. From a con- 

 sideration of the present orbital elements of Mercury 

 and their evolution, he finds that the requisite order of 



GEOLOGICAL TIME SCALE, BASED ON THE LEAD-URAXIUM RATIOS OF RADIO-ACTIVE MINERALS 



i 



1 The sign (?) indicates that the geological age of the rocks in which the mineral occurs has not been unequivocally determmed. 

 In any such case the numerical age deduced from lead-ratios cannot be considered as a contribution to a geological time scale, 

 but according to the place it occupies in the latter it may conversely be used as evidence of the geological age of the mineral and 

 rock in question. 



